[Ada] Fix undefined reference at link time for array size symbol

[Eric Botcazou]

This fixes a link failure for an array object declared with an array of array 
type under very specific circumstances, as the compiler doesn't consistently 
consider that the size of the array type is constant, depending on the unit 
which is being compiled.

I need to export the skip_simple_constant_arithmetic function I wrote a few 
years ago, so I've moving it next to its model skip_simple_arithmetic and 
tidying up the implementation of the latter (but no functional changes).

Tested on x86_64-suse-linux, applied on mainline.


2013-04-11  Eric Botcazou  <ebotcazou@adacore.com>

	* stor-layout.c (skip_simple_constant_arithmetic): Move to...
	* tree.c (skip_simple_constant_arithmetic): ...here and make public.
	(skip_simple_arithmetic): Tidy up.
	* tree.h (skip_simple_constant_arithmetic): Declare.


2013-04-11  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (elaborate_expression_1): Skip only constant
	arithmetics when looking for a read-only variable in the expression.


2013-04-11  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/array23.adb: New test.
	* gnat.dg/array23_pkg[123].ads: New helpers.


-- 
Eric Botcazou
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